The present invention relates generally to the field of pulse detectors in which a missing or extra pulse in a variable frequency periodic pulse train is detected. More specifically, the present invention relates to such pulse detectors which are adaptable for use in engine control systems in which a variable frequency periodic pulse train produced by sensing the passage of fixed projections or notches on a wheel rotated by an engine crankshaft, especially wherein the detection of a missing or extra pulse is used to identify a predetermined engine crankshaft reference rotational position.
In prior engine control systems, it is typically necessary to identify a reference rotational position of the engine crankshaft in order to initiate the proper sequencing of engine control signals to the engine cylinders for implementing spark/dwell control and/or fuel injection control. Also, for engine control it may be necessary to sense the speed of rotation of the crankshaft and provide pulses which occur at regular closely spaced intervals of crankshaft rotation. Typically such systems utilize a sensor to sense the passage of a number of projections on a wheel rotated by an engine crankshaft and in some systems the occurrence of a reference rotational position of the engine crankshaft is indicated by the absence of one of these projections resulting in a corresponding absence of a periodic pulse being provided by the sensor. Missing pulse detectors have been used in such engine control systems wherein these detectors attempt to determine when a pulse is absent in a variable frequency periodic pulse train provided in accordance with the rotation of an engine crankshaft. Typically all of these prior missing pulse detectors develop either an analog or digital signal related to the time duration between successive pulse train pulses, and these prior detectors conclude that a missing pulse has occurred when the time duration between successive pulses exceeds by a predetermined amount the time duration between preceding successive pulses. While this method is feasible for determining the existence of a missing pulse in a pulse train, clearly this technique is subject to misinterpreting any substantial deceleration in the occurrence rate of the pulse train pulses as the existence of a missing pulse, especially when this deceleration is abrupt. None of the prior pulse detectors apparently attempts to minimize this potential erroneous mode of operation in which a reduction in the frequency of the periodic pulse train can be mistaken for the occurrence of a missing pulse train pulse.
In some systems, it is possible to determine a reference position by providing an additional projection on a rotating wheel having a number of spaced projections, rather than by removing one of the projections to create a missing pulse. In such systems, again it is believed that the detectors utilized therewith determine the existence of this extra pulse corresponding to a reference location by comparing the time duration between pulses with the time duration that existed between preceding pulses. Such systems are likely to misinterpret an acceleration in the frequency of the periodic pulse train pulses as the existence of an extra pulse, and the prior systems have no satisfactory way of distinguishing between the occurrence of a true extra pulse and an abrupt acceleration of the frequency of the periodic pulse train.